1. Field of the Invention
The present invention relates to velocity measurement systems and, more particularly, to sonar systems performing bottom tracking.
2. Description of the Prior Art
A current profiler is a type of sonar system that is used to remotely measure water velocity over varying ranges. Current profilers are used in freshwater environments such as rivers, lakes and estuaries, as well as in saltwater environments such as the ocean, for studying the effects of current velocities. The measurement of accurate current velocities is important in such diverse fields as weather prediction, biological studies of nutrients, environmental studies of sewage dispersion, and commercial exploration for natural resources, including oil.
Typically, current profilers are used to measure current velocities in a vertical column of water for each depth "cell" of water up to a maximum range, thus producing a "profile" of water velocities. The general profiler system includes a transducer to generate pulses of sound (which when downconverted to human hearing frequencies sound like "pings") that backscatter as echoes from plankton, small particles, and small-scale inhomogeneities in the water. The received sound has a Doppler frequency shift proportionate to the relative velocity between the scatters and the transducer.
The physics for determining a single velocity vector component (v.sub.x) from such a Doppler frequency shift may be concisely stated by the following equation: ##EQU1## In equation (1), c is the velocity of sound in water, about 1500 meters/second. Thus, by knowing the transmitted sound frequency, f.sub.T, and declination angle of the transmitter transducer, .theta., and measuring the received frequency from a single, narrowband pulse, the Doppler frequency shift, f.sub.D, determines one velocity vector component. Relative velocity of the measured horizontal "slice", or depth cell, is determined by subtracting out a measurement of vessel earth reference velocity, v.sub.e. Earth reference velocity can be measured by pinging the ocean bottom whenever it comes within sonar range or by a navigation system such as LORAN or GPS.
Commercial current profilers are typically configured as an assembly of four diverging transducers, spaced at 90.degree. azimuth intervals from one another around the electronics housing. This transducer arrangement is known in the technology as the Janus configuration. A three beam system permits measurements of three velocity components, v.sub.x, v.sub.y and v.sub.z (identified respectively as u, v, w in oceanographic literature) under the assumption that currents are uniform in the plane perpendicular to the transducers mutual axis. However, four beams are often used for redundancy and reliability. The current profiler system may be attached to the hull of a vessel, remain on stationary buoys, or be moored to the ocean floor.
Of particular importance to the vessel mounted current profiler is the accurate determination of vessel velocity. The earth reference water velocities can then be calculated by subtracting out the vessel velocity. As is well-known, the movement of the vessel with respect to the earth is based on establishing at least two fixed reference points over a period of time. In a current profiler, one common technique to find the bottom is to interleave a bottom range pulse with the current velocity pulses. The bottom range pulse is generally of a longer duration than other pulses so as to fully ensonify the bottom. The length of the pulse is chosen according to the assumed maximum depth and the angle subtended by the transducer.
In some existing current profilers the decision-making for bottom detection has been based on a simple comparison between received signal amplitude and a threshold value. While performing reasonably well, these systems may produce "false bottoms" as a result of life layers, e.g., plankton, or schooling fish which offer alternative sources of acoustic reflection. Thus, it will be readily appreciated that false bottoms, located at ranges from the transducer that are less than the range to the actual bottom, lead to inaccurate range and velocity measurements.
In other sonar systems including, for instance, depth sounders, bottom mapping sonars, sidescan sonars, speed logs and correlation logs, matched filtering techniques (or equivalent correlation techniques) have been used to minimize the number of false bottoms. Matched filtering is a technique that applies a signal to a linear filter so as to statistically determine the existence of a signal of interest. The matched filter is well-known in the relevant technology and descriptions thereof can be found in "Statistical Theory of Signal Detection", Second Edition, Carl W. Helstrom, Pergamon Press, 1968, pp. 112-115, and "Modulation, Noise, and Spectral Analysis", Philip F. Panter, McGraw-Hill, 1965, pp. 730-733, which are hereby incorporated by reference.
Among the prior art, the patents to Backman, Jr. (U.S. Pat. No. 4,054,862), Morgera (U.S. Pat. No. 4,207,620) and Kits van Heyningen (U.S. Pat. No. 4,404,665) discuss the matched filter approach to bottom detection. However, these approaches compare the output of the standard matched filter with a predetermined threshold value and thus they may still detect false bottoms. In addition, none of these systems account for the principal sources of signal loss in water.
There are two major sources of signal loss that may produce errors whenever a sonar echo is compared with an absolute reference value or threshold. First, unlike electronic emissions propagating through air, sound waves traveling in water are subject to water absorption losses due to thermal effects. Second, due to signal spreading (intuitively akin to the spreading ripples which emanate from a rock thrown into a pond), the strength of the transmitted signal is inversely proportional to the square of the range. Hence, these sources of signal loss will also affect any comparison of a filtered signal with a threshold value.
Accordingly, more accurate sonar systems to detect the bottom of a body of water are desired. In particular, a sonar system that minimizes the detection of false bottoms will improve the quality of vessel and water velocities. It would be a further improvement if the sonar system could compensate for signal losses due to water absorption and spreading.